Reliability vs Availability Reliability Is anything broken Availability

Reliability vs Availability • Reliability: Is anything broken? • Availability: Is the system still available to the user? Lecture 4 1

RAID • Redundant Array of Inexpensive Disks – Higher throughput – Ability to recover from failures • • • Disk Array (Availability reduced to 1/N( Data Integrity Striping MTTR MTTF Lecture 4 2

Manufacturing Advantages of Disk Arrays Disk Product Families Conventional: 4 disk ” 3. 5 ” 5. 25 ” 10 designs Low End ” 14 High End Disk Array: 1 disk design ” 3. 5 Lecture 4 3

Replace Small # of Large Disks with Large # of Small Disks! (1988 Disks( IBM 3390 (K( IBM 3. 5" 0061 x 70 20 GBytes 320 MBytes 23 GBytes 97 cu. ft. 0. 1 cu. ft. 11 cu. ft. 3 KW 11 W 1 KW Data Rate 15 MB/s 1. 5 MB/s 120 MB/s I/O Rate 600 I/Os/s 55 I/Os/s 3900 IOs/s MTTF 250 KHrs ? ? ? Hrs Cost 250 K$ 2 K$ 150 K$ Data Capacity Volume Power large data and I/O rates Disk Arrays have potential for high MB per cu. ft. , high MB per KW reliability? Lecture 4 4

Array Reliability • Reliability of N disks = Reliability of 1 Disk / N 50, 000 Hours / 70 disks = 700 hours Disk system MTTF: Drops from 6 years to 1 month! • Arrays (without redundancy) too unreliable to be useful! Hot spares support reconstruction in parallel with access: very high media availability can be achieved Lecture 4 5

Redundant Arrays of Disks • Files are "striped" across multiple spindles • Redundancy yields high data availability Disks will fail Contents reconstructed from data redundantly stored in the array Capacity penalty to store it Bandwidth penalty to update Mirroring/Shadowing (high capacity cost( Techniques: Horizontal Hamming Codes (overkill( Parity & Reed-Solomon Codes Failure Prediction (no capacity overhead(! Vax. Sim. Plus — Technique is controversial Lecture 4 6

Redundant Arrays of Disks RAID 1: Disk Mirroring/Shadowing recovery group • Each disk is fully duplicated onto its "shadow" Very high availability can be achieved • Bandwidth sacrifice on write: Logical write = two physical writes • Reads may be optimized • Most expensive solution: 100% capacity overhead Targeted for high I/O rate , high availability environments Lecture 4 7

Redundant Arrays of Disks RAID 3: Parity Disk 10010011 11001101 10010011. . . logical record Striped physical records P 1 0 0 1 1 0 0 1 1 0 0 • Parity computed across recovery group to protect against hard disk failures 33% capacity cost for parity in this configuration wider arrays reduce capacity costs, decrease expected availability, increase reconstruction time • Arms logically synchronized, spindles rotationally synchronized logically a single high capacity, high transfer rate disk Targeted for high bandwidth applications: Scientific, Image Processing Lecture 4 8

Redundant Arrays of Disks RAID 5+: High I/O Rate Parity A logical write becomes four physical I/Os Independent writes possible because of interleaved parity Reed-Solomon Codes ("Q") for protection during reconstruction Targeted for mixed applications D 0 D 1 D 2 D 3 P D 4 D 5 D 6 P D 7 D 8 D 9 P D 10 D 11 D 12 P D 13 D 14 D 15 P D 16 D 17 D 18 D 19 D 20 D 21 D 22 D 23 P . . Disk Columns. . . Increasing Logical Disk Addresses Stripe Unit Lecture 4 9

Problems of Disk Arrays: Small Writes RAID-5: Small Write Algorithm 1 Logical Write = 2 Physical Reads + 2 Physical Writes D 0' new data D 0 D 1 D 2 D 3 old data . 1)Read( P old . 2)Read( parity + XOR . 3)Write( D 0' D 1 . 4)Write( D 2 D 3 P' Lecture 4 10

Subsystem Organization host adapter array controller manages interface to host, DMA control, buffering, parity logic physical device control striping software off-loaded from host to array controller no applications modifications no reduction of host performance single board disk controller often piggy-backed in small format devices Lecture 4 11

Summary: A Little Queuing Theory System Queue Proc server IOC Device • Queuing models assume state of equilibrium: input rate = output rate • Notation: r Tser u Tq Tsys Lq Lsys average number of arriving customers/second average time to service a customer (tradtionally � = 1/ Tser ) server utilization (0. . 1): u = r x Tser average time/customer in queue average time/customer in system: Tsys = Tq + Tser average length of queue: Lq = r x Tq average length of system : Lsys = r x Tsys • Little’s Law: Lengthsystem = rate x Timesystem (Mean number customers = arrival rate x mean service time( Lecture 4 12

Summary: Redundant Arrays of Disks (RAID) Techniques • Disk Mirroring, Shadowing (RAID 1( Each disk is fully duplicated onto its "shadow" Logical write = two physical writes 100% capacity overhead • Parity Data Bandwidth Array (RAID 3( Parity computed horizontally Logically a single high data bw disk • High I/O Rate Parity Array (RAID 5( 1 0 0 1 1 1 0 0 1 1 0 0 1 0 Interleaved parity blocks Independent reads and writes Logical write = 2 reads + 2 writes Parity + Reed-Solomon codes Lecture 4 13

Homework #3 • Problem 6. 5 and 6. 6 • Due date: June 26, 2001 Lecture 4 14

Assignment #1 • Explain RAID 0 -6 in greater detail. • Due date: July 3, 2001 Lecture 4 15